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Home My WebLink AboutB13-0260 Geologic Hazard Report.pdf CTL I THOMPSON July 22, 2013 Suman Architects 141 East Meadow Drive, Suite 211 Vail, Colorado 81657 Attention: Michael Suman, AIA Subject: Geologic Hazards Evaluation 2470 Bald Mountain Road Vail, Colorado Project No. GS05775-105 At your request, I've prepared this letter to summarize my investigation of the geologic hazards that may affect the subject site. The work was conducted to fulfill the scope of work described in our proposal (DN-13-0424, dated July 11, 2013). 1 conducted the field work for this project on July 17, 2013. The weather was clear and mild. Site Conditions and Planned Construction As I understand the project, the existing home at 2470 Bald Mountain Road (also known as Lot 19 Block 2 Vail Village 13th Filing) will be demolished and removed to allow construction of a new home. It appears the existing home dates from the 1970's. You provided plans for the Stockton Residence dated July 9, 2013. Plans show a ranch-style home over a basement that will walk-out at the southwest corner. A 3-car garage is planned at the north end of the home, with a new drive and heated auto court facing west. The lot slopes moderately and then gently to the southwest toward the ad- jacent Lot 20 and open-space beyond. Existing grades will not be modified substantially. An existing car-parking area at the northeast corner of the lot will be removed, along with the timber retaining wall that has failed. Drainage is generally southwest from the lot above (Lot 18) into a swale at the Lot 19/20 line. Bald Mountain Road is north, with a steep slope beyond. Photos 1, 2 and 3 show the property conditions I observed. 234 Center Drive I Glenwood Springs, Colorado 81601 1 Telephone: 970-945-2809 Fax: 970-945-7411 fI - s � '��, 1r. 5 I Photo 1—View to the SE toward existing residence.Existing car parking area is at left. A 1 t y µr_S a Photo 2—View to the NE from front of house towards car parking area to be removed. SUMAN ARCHITECTS Z 2470 BALD MOUNTAIN ROAD CTLIT PROJECT NO.GS05775-105 Y:\GS05775.000\105\3.Letters\L1\GS05775-105-L1.docx yJ, L�5 Photo 3-View to the west showing Bald Mountain Road and slope above Lot 19 which is left of the white pickup truck. Geologic Setting Based on review of the Geologic Map of the Vail East Quadrangle by Kel- logg, Bryant and Redsteer (USGS MF-2375, dated 2003) 2470 Bald Mountain Road is situated on a colluvial apron and debris fan below a slope underlain by the lower member of the Paleozoic age Minturn formation, interlayered sand- stone, conglomerate and shale. The sedimentary formation is bedded with about a 10 degree dip to the south, out of the slope. The map excerpt below is from the 2003 USGS map. Photo 4 shows an outcrop of this formation about 400 feet east of Lot 19. No seeps were noted at the time of my observation. SUMAN ARCHITECTS 3 2470 BALD MOUNTAIN ROAD CTLIT PROJECT NO.GS05775-105 Y:\GS05775.000\105\3.Letters\L1\GS05775-105-L1.docx P n-1 l 5 ` Excerpt from 2003 USGS Vail East Quadrangle Geologic Map Photo 4—Outcrop of the lower member of the Minturn formation Geologic Hazards In their Resolution 13, on October 17, 2000 the Town of Vail adopted Hazard Maps for Rockfall, Debris Flow and Avalanche. These are necessarily large-scale general maps. The extreme north side of the subject site is shown within a high severity rockfall hazard region as shown on the map excerpt below. SUMAN ARCHITECTS 4 2470 BALD MOUNTAIN ROAD CTLIT PROJECT NO.GS05775-105 Y:\GS05775.000\105\3.Letters\L1\GS05775-105-L1.docx d' t Hazard Zones [] Rockfall Hazard with Approved Mitigation High Severity Rockfall 0 Medium Severity Rockfall Based on my recent site observations, I believe the rockfall hazard for Lot 19 is actually low. I do not believe further mitigation with respect to rockfall is neces- sary for Lot 19. 1 based this conclusion on my observations: • The slope north of the lot has relatively few and low outcrop cliffs to shed rocks, compared to other areas, particularly to the west. There is a large disconnected rock slab on the slope, however it is unlikely to roll or tumble onto the lot. See Photo 5 below. • The overall slope for the first 400 horizontal feet above Bald Moun- tain Road is about 50%, somewhat steeper than the threshold for rockfall of about 40%. The slope above that is about 35 percent, less than the threshold for rockfall. The slope is heavily vegetated with brush and small trees, although that is generally not consid- ered a mitigating factor because a potential fire could remove vege- tation. A few small rocks were noted on the retaining wall above Bald Mountain Road that have evidently come to rest recently. These were under a foot in maximum dimension. No fresh rockfall was noted in the borrow ditch below the retaining wall, nor was any noted on the lots on the south side of Baldy Mountain Road west and east of the subject lot. The road, lawn, garage and auto court SUMAN ARCHITECTS 5 2470 BALD MOUNTAIN ROAD CTLIT PROJECT NO.GS05775-105 YAGS05775.000\105\3.Letters\L1\GS05775-105-L1.docx area should provide adequate run-out area in the unlikely event rockfall were go beyond the retaining wall. w t 'p a tl Ms" t Photo 5—View to the north showing the slope above Lot 19.Note few low outcrops and heavy brush vegetation Lot 19 is shown on Vail's Debris Flow Hazard Map as being exposed to moderate hazard debris flow, as shown on the map excerpt below. Hazard Zones High Hazard Debris Flow 0 Moderate Hazard Debris Flow -High Hazard Debris Avalanche SUMAN ARCHITECTS G 2470 BALD MOUNTAIN ROAD CTLIT PROJECT NO.GS05775-105 Y:\GS05775.000\105\3.Letters\L1\GS05775-105-L1.docx I believe proposed grading is relatively insignificant and will not materially change flow patterns on the lot. With respect to potential debris flow on the lot, there are some mitigating factors present that likely were not there when the mapping adopted in 2000 was prepared. The retaining wall and curb-gutter system bounding the 30-foot wide Bald Mountain Road should provide a course for storm runoff and debris flow as it travels down the road to the regional deten- tion basin southwest of the subject lot. Should debris or mudflow jump the curb in front of Lot 19, it would likely flow across the lot toward Lot 20. It appears the planned home appears to have low risk with respect to debris flow. The storm inlet in the drainage about 300 feet east of the lot contained some small boulders but no evidence of mud or debris. At the time of my obser- vation, the inlet was partially clogged with rocks as shown on Photo 6 below. This should be maintained every spring at the least. u / w Photo 6-View to the west at storm inlet located about 300 feet east of Lot 19. Note boulders but no evidence of mudflow. Vail's Avalanche Hazard Map shows the site is not subject to avalanche hazard. Other geologic hazards common in the mountainous regions of Colorado include landslides, unstable or potentially unstable slopes, ground subsidence, and expansive soil and rock. Lot 19 is not exposed to these hazards. I found no signs that the slope above the site is unstable or potentially unstable, nor did I or previous investigators find landslides. Ground subsidence is not common in the formation underlying the site. Expansive soil and rock is not present. Regional issues of seismicity and radioactivity are believed to be at a low level, similar to the rest of the Vail area. In summary, I believe 2470 Bald Mountain Road is not SUMAN ARCHITECTS 7 2470 BALD MOUNTAIN ROAD CTLIT PROJECT NO.GS05775-105 Y:\GS05775.000\105\3.Letters\L1\GS05775-105-L1.docx subject to geologic hazards that preclude the planned construction or require mitigation to bring risk to a low level. Although the site is exposed to a low risk from debris or mudflow, this hazard is small for property damage and is not believed to be a risk to life. Preventing mudflow onto Lot 19 would require coor- dinated effort with neighboring Lots 18 and 20, and may not be warranted. Limitations The conclusions and recommendations presented in this letter are based on the proposed construction as currently planned. Revisions in the planned construction could affect our recommendations. We should be contacted if plans change to review and revise our recommendations, if necessary. We believe this investigation was conducted in a manner consistent with that level of care and skill ordinarily used by engineering geologists practicing in this area at this time. No warranty, express or implied, is made. If we can be of further service in discussing the contents of this letter, or in the analysis of the influence of the geologic conditions on the design of the building or any other aspect of the proposed construction, please call. tic � +r•.gat,ah, CTL TH P I►� C,y ��j 02 i ',� All David A. Glater, t G.PG pry ' Principal Geologi 0e DAG/nt " (3 copies) Via e-mail- Michaelosumanarchitects.com SUMAN ARCHITECTS 8 2470 BALD MOUNTAIN ROAD CTLIT PROJECT NO.GS05775-105 YAGS05775.000410 I LelterMOL GS05 7 7 5-1 05-1_1 dou